Superconductors are also called superconducting materials. It generally refers to a conductor whose resistance is zero at a certain temperature. In the experiment, if the measured value of the conductor resistance is lower than 10 to the -25th power Ω, the resistance can be regarded as zero. Superconductors not only have the characteristics of zero resistance, another important feature is complete diamagnetism.
Superconductor applications can be divided into three categories: strong current applications, weak current applications, and diamagnetic applications. High-power applications are high-current applications, including superconducting power generation, transmission, and energy storage; weak-power applications are electronic applications, including superconducting computers, superconducting antennas, and superconducting microwave devices; antimagnetic applications mainly include maglev trains and thermonuclear Fusion reactor, etc.
The discovery of superconductors came from an unexpected move by Dutch scientist Heike Kamelin Onnes in 1911. In 1908, thanks to the development of cryogenic technology, Ang from the Leiden Cryogenic Laboratory of the University of Leiden in the Netherlands Professor Nes improved the laboratory equipment with great efforts. By using compressed nitrogen throttling pre-cooling hydrogen and hydrogen compression throttling pre-cooling helium, the helium was finally liquefied by compression throttling method, obtaining a low temperature of 4.2K. Successfully liquefied the last “permanent gas”-helium.
The breakthrough in low temperature research has laid the foundation for the discovery of superconductors. Intensive efforts by Annes found in 1911 that at a low temperature of 4.3K, the resistance of platinum remained constant, rather than increasing after passing a minimum value.
Therefore, Annes believed that the resistance of pure platinum should disappear at the temperature of liquid helium. In order to verify this conjecture, Onnes chose mercury that was easier to purify as the experimental object. First, Onnes cooled mercury to minus 40 degrees Celsius to solidify the mercury into a line; then using liquid helium to reduce the temperature to around 4.2K, and applying a voltage across the mercury line; when the temperature was slightly below 4.2K ( At -269 ℃, the formula for converting Kelvin to Celsius is -273 Kelvin, because the absolute zero degree is -273 degrees), the resistance of mercury suddenly disappeared, showing a superconducting state, and later he found many metals Both the alloy and the alloy have the characteristics of losing resistance at low temperature similar to the above-mentioned mercury. Due to its special conductivity, Camorin-Onnis calls it the superconducting effect.
After him, people began to call the conductor in superconducting state “superconductor”. In 1933, the Netherlands ’Meissner and Orsfeld jointly discovered another extremely important property of a superconductor. When the metal is in a superconducting state, the magnetic induction intensity in this superconductor is zero, but the original existence in the body The magnetic field is squeezed out. Experiments on single crystal tin balls found that when the tin balls transitioned to the superconducting state, the magnetic field around the tin balls suddenly changed, and the magnetic field lines seemed to be repelled out of the superconductor at once. “Sner effect”, which is what we said at the beginning of the semiconductor is completely diamagnetic.
Therefore, the Meissner effect and the zero-resistance phenomenon are the two major factors to experimentally determine whether a material is a superconductor. Superconductivity has some important practical applications, such as MRI in hospitals, high-energy accelerators, magnetic confinement nuclear fusion devices, etc., but for a long time, a major bottleneck restricting the wide application of superconductors is that the best superconductors Use liquid helium or liquid nitrogen to cool it (usually to -250 ℃).
Theoretical physicists are also trying to solve the mystery of superconductors. Until 1957, three physicists proposed the BCS theory, based on the near-free electron model, which explained the conventional superconductor under the premise of weak electron-phonon interaction. The microscopic theory of superconductivity, and thus won the 1972 Nobel Prize in Physics.
The American physicist McMillan also discovered that there is a limit temperature of about 39K in BCS theory. Any substance higher than this temperature cannot form a superconducting state. This discovery is called the McMillan limit, and this limit has hit people Confidence, because such a low temperature is difficult to use in practice.
Because human pursuit is to achieve normal temperature superconductivity, such a low temperature is really difficult to apply in real life, the difficulty is too high, and the investment is too high. At present, scientists are still exploring the field of high temperature superconductivity. High temperature superconductors are not what most people think The high temperature of hundreds of thousands is only a lot higher than the ultra-low temperature required by the original superconductor, but it is also more than minus hundreds of degrees Celsius. In the superconductors studied by humans, the temperature has increased a lot, so it is called high temperature superconductor.
In 1987, physicists Wu Maokun and Zhu Jingwu raised the critical superconducting temperature above 90K on yttrium-barium-copper-oxygen materials, and the “temperature barrier” (77K) of liquid nitrogen was also broken.
This is the first time in history that the superconducting temperature has been raised from 30K to 90K (minus 183 degrees Celsius) beyond the boiling temperature “temperature barrier” of liquid nitrogen, breaking through the bottleneck of physics research more than 70 years since 1911, which is the critical temperature Materials higher than 77K have been defined as high-temperature superconductors. Since then, many scientists have begun to try to break the limits of McMillan and strive to find “high-temperature superconductors.”
In addition, the use of cheaper liquid nitrogen in this experiment will greatly reduce the application cost of superconductivity, making large-scale application of superconductivity and in-depth scientific research possible.
At present, the highest temperature of the material to reach the superconducting state is about 133K, and this material is copper oxide, which was discovered in the 1980s. However, for oxide-type high-temperature superconductors, because the microstructure is very complex, the structure is often difficult to adjust, and it is difficult to study at the microscale, so it is difficult to find its superconducting mechanism; and ultrahigh voltage superconductors are more difficult to study Unable to achieve practical application.
If any material can exhibit superconductivity at room temperature, it can bring revolutionary changes to the field of energy transmission, medical scanners and transportation.
But the Academy of Sciences has never made a breakthrough, and the staff got an unexpected result that excited them in 2016. They used two pieces of graphene to construct a sandwich-like structure. After inserting some calcium atoms into the graphene sheet, they were surprised to find that this structure achieved superconductivity! In other words, the material constructed in this way can achieve zero resistance.
Why does superconductivity occur at the magic angle of 1.1 °? Based on the tight-binding calculation method of the energy band structure, this magical “magic angle” can be calculated based on the change of the double-graphene energy band diagram relative to the angle. When the graphene layers are twisted at an angle, the electron orbits in them will re-hybridize and change the hybrid energy. The result of the competition and competition between the hybrid energy and the electronic kinetic energy has created the phenomenon of “magic” in this perspective. In other words, the torsion angle θ gradually increases, and the hybrid energy also increases. When the Fermi speed decreases from the Fermi speed 0 = 106m / s in a single-layer graphene to = 0, the corresponding torsion angle θ0 is It is called “Magic Horn”. At this time, it corresponds to that the hybrid energy is equal to the kinetic energy of electrons, that is, 2 = ℏ0θ0, and the magic angle θ0 = √3ℏ0 = 1.08 °, which is about 1.1 °. In such a case, the corresponding energy band diagram becomes an almost flat insulator energy band diagram, that is, a phenomenon similar to a Mott insulator is produced, and the insulation and superconductivity can be converted to each other in one step.
Although the system still needs to be cooled to 1.7K, due to the simple structure of graphene, the fabricated device is more suitable for research than copper oxide. If high-temperature superconductivity can be achieved in a simple structure such as graphene, its application value and research The value is extraordinary, and scientists believe that graphene is more likely to achieve room temperature superconductivity than copper oxide
To better understand copper oxide, physicists have been groping for 30 years in the dark. And the latest discovery may have just ignited a beam of light for physicists.
Graphene is an insulator or superconductor, so what are its superconducting properties in the future?
When rotating at a “magic angle”, graphene sheets can form insulators or superconductors.
Since its discovery in 2004, scientists have discovered that the single layer of carbon atoms in the lace and honeycomb-like graphene, graphene, is not only the thinnest material known in the world, but is hundreds of times stronger than steel and more conductive than copper. it is good.
When rotated at a magic angle, the two pieces of graphene exhibit non-conductive behavior, similar to a peculiar material called Mott insulator.
When a small amount of electrons were added to the graphene superlattice when a voltage was applied, it was found that to some extent, the electrons broke through the initial insulation state and flowed without resistance, just like through a superconductor. A large-scale interpretation of the moiré pattern formed when one graphene lattice rotates slightly at a “magic angle” relative to another graphene lattice.
The ability of a material to conduct electricity is usually expressed in terms of energy bands.
A single energy band represents the energy range of the electronic energy of the material.
There is an energy gap between the bands. When a band is filled, electrons must contain additional energy to overcome this energy gap in order to occupy the next empty band.
If the last occupied energy band is completely filled with electrons, the material is considered an insulator.
On the other hand, conductors such as metals exhibit partially filled energy bands, with empty energy states in which electrons can move freely.
However, Mott insulators are a material that is electrically conductive from their belt structure, but when measured, they behave like insulators.
Specifically, their energy bands are half full, but due to strong electrostatic interactions between electrons (for example, same-sex charges repel each other), this material is not conductive.
The half-full band is essentially split into two miniature, almost flat bands, and the electrons completely occupy one band and leave another empty band, so it behaves as an insulator.
All electrons are blocked, so it is an insulator, because there is a strong repulsion between electrons, so nothing can flow.
It turns out that most of the parent compounds of high-temperature superconductors are Mott insulators. In other words, methods have been found to manipulate the electrical properties of Mott insulators so that they become superconductors at a relatively high temperature of around 100 Kelvin.
When studying the electronic properties of graphene, a graphene sheet was peeled from the graphite, and then half of it was carefully peeled off with a glass slide coated with an adhesive polymer and boron nitride insulating material, thereby creating two layers Superlattice. Then they rotated the slide very slightly and glued the lower half of the graphene sheet to the slide. In this way, the electrode is connected to each device, the passing current is measured, and then the resistance of the device is drawn according to the original current passed. It is found that the rotation angle of the graphene superlattice is 1.1 degrees, which is predicted as a “magic” Angle ”, its electronic structure is similar to a flat belt structure, similar to a Mott insulator, regardless of the momentum, all electrons carry the same energy. For electrons, this means that even if they occupy a half-full energy band, an electron does not have more energy than other electrons to make it move in this energy band. Therefore, although such a semi-filled tape structure should work like a conductor, it behaves like an insulator, more precisely, a Mott insulator. If electrons can be added to these superlattices similar to Mott insulators, a single electron can be combined with other electrons in graphene, allowing them to flow where they could not flow before. Throughout the process, we continued to measure the resistance of this material and found that when a certain number of electrons are added, the current does not dissipate energy, just like a superconductor. The current can flow freely without wasting energy, which shows that graphene can become a superconductor. More importantly, the performance of graphene can be adjusted to become an insulator or superconductor, and any state between the insulator and the superconductor.
The characteristic of this superconductor is room temperature superconductivity, which is revolutionary!